1. Field of the Invention
This invention relates to a catalyst for the purification of exhaust gases capable of purifying exhaust gases from internal combustion engines such as automotive engines by removing carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NOx).
2. Prior Art
As a conventional catalyst for the purification of automobile exhaust gases, there has generally been known a catalyst comprising a support substrate, a catalyst carrier layer formed on the surface of the support substrate and catalyst ingredients carried on the catalyst carrier layer, and so far, there have been demonstrated various kinds of catalysts for the purificaion exhaust gases aiming at efficient purification.
For example, some techniques using cerium have been disclosed in Japanese Examined Patent Publication (KOKOKU) Nos. 20307/1983 and 41775/1984 and Japanese Unexamined Patent Publication (KOKAI) No. 90695/1984. In the catalysts of these techniques using cerium, cerium is present in the oxide form. The cerium improves the purification property by discharging or storing oxygen (oxygen storing capability) according to the reaction shown in formula (1) and by controlling the oxidation of CO and HC, and the reduction of NOx. EQU CeO.sub.2 .revreaction.CeO.sub.2-x +(x/2)O.sub.2 ( 1)
It has been known that the reaction shown in formula (1) occurs on the surface of the cerium oxide particles. However, in the aforesaid conventional catalysts for the purification of exhaust gases, the total surface area of the cerium oxide particles was reduced by the growth of the cerium oxide particles when the conventional catalysts were used at elevated temperatures 800.degree. C. or more. Accordingly, there has arised a disadvantage that the purification property is degraded due to the deterioration of the oxygen storing capability.
Also, some techniques using other rare earth and transition metals simultaneously with cerium for aiming at stabilization of activated alumina, were disclosed in Japanese Examined Patent Publication (KOKOKU) No. 7537/1985, Japanese Unexamined Patent Publication (KOKAI) Nos. 18180/1973, 63133/1982 87839/1982, U.S. Pat. No. 3,003,020, U.S. Pat. No. 3,951,860 and U.S. Pat. No. 4,170,573. For example, techniques disclosed in Japanese Examined Patent Publication (KOKOKU) No. 7537/1985 and Japanese Unexamined Patent Publication (KOKAI) No. 63133/1982 have used cerium with lanthanum (La) and formed a composite oxide shown in formula (2). EQU Ce.sub.1-x La.sub.x O.sub.2- (x/2) (2) EQU (0.3.ltoreq.X.ltoreq.0.5)
In the catalysts for the purification of exhaust gases of the above techniques, a lattice defect with oxygen vacancies is formed in the fluorite structure of the composite oxide, so that the oxygen storing capability having durability can be achieved.
However, in those catalysts for the purification of exhaust gases, lanthanum forms LaAlO.sub.3 at elevated temperatures, particularly at over 800.degree. C. Therefore, a surface area of alumina catalyst carrier layer is reduced and transformed by the LaAlO.sub.3 formation. Further, the growth of catalyst ingredient particles arises. Consequently, there have arisen disadvantages that the efficiency of purification is deteriorated. In short, those catalysts have showed poor durability at elevated temperatures.
In U.S. Pat. No. 4,170,573, a catalyst having durability at elevated temperatures is obtained by carrying catalyst ingredients after heating a catalyst support having a catalyst carrier layer comprising cerium and lanthanum at elevated temperatures 983 to 1317.degree. C. However, this catalyst has not shown the sufficient durability at elevated temperatures since the formation of LaAlO.sub.3 is not prevented and the surface area is reduced.